Polyester-based aqueous coating composition

Abstract

Polyester-based aqueous coating compositions comprising a mixture of a carboxyl-functional polyester resin (A), a water-insoluble epoxy resin (B), and a hydrophobic solvent (C), the mixture being neutralized with neutralizer (D) and dispersed or dissolved in water. The compositions have excellent film forming and processing characteristics, and are particularly useful in coating the interior surface of cans and as automobile coatings.

Description

1. Field of the InventionThis invention relates to crosslinkable aqueous coating compositions which exhibit excellent film-forming and curing characteristics, in terms of processability and safety. The compositions can be used for coating the interior surfaces of cans. Moreover, these crosslinkable aqueous coating compositions are capable of forming films having excellent hardness, and accordingly can be used as primers or finish coats for automotive or industrial applications.2. Description of the Prior ArtIn the past, aqueous coating compositions have been made from the reaction product of a high acid value acrylic resin and a high molecular weight bisphenol A based epoxy resin. These compositions have been used for coating the interior surface of cans. However, recently questions have been raised about bisphenol A, and effort has been directed to coating compositions in which bisphenol A does not elute.Polyester resins have previously been proposed for use as a base resin in aque...

AI Technical Summary

Benefits of technology

Examples of polybasic acids which can be used in the present invention, mainly in the preparation of the polyester resin, include dicarboxylic acids such as phthalic acid, phthalic anhydride, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, tetrahydrophthalic anhydride, hexahydrophthalic acid, hexahydrophthalic anhydride, hexahydroisophthalic acid, hexahydroterephthalic acid, 4-carbonylhexahydrophthalic acid, 4-carbonylhexahydrophthalic anhydride, 3-carbonyltetrahydrophthalic acid, 3-carbonyltetrahydrophthalic anhydride, succinic acid, succinic anhydride, fumaric acid, adipic acid, azelaic acid, sebacic acid, maleic acid, maleic anhydride and lower alkyl esters of these dicarboxylic acids. In addition to these dicarboxylic acids, monobasic acids such as benzoic acid, crotonic acid, p-t-butyl benzoic acid and other various fatty acids can be used. Further, trivalent or higher polycarboxylic acids such as trimellitic acid, trimellitic anhydride, methylcyclohexene tri-carboxylic acid, pyromellitic acid, pyromellitic anhydride and butane tricarboxylic acid can also be used. Among these polybasic acids, it is important that the polyvalent aromatic carboxylic acid comprise at least about 80 mol %, preferably more than 90 mol %, based on the total polybasic acid components. This high concentration of the polyvalent aromatic carboxylic acid has been found to contribute good hydrolysis stability resistance.

Examples of the polyalcohols which can be used in the present invention, mainly in the preparation of the polyester resin, include dihydric alcohols including aliphatic glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,2 butanediol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, 2-methylpropane diol, 3-methylpentane diol, 1,4-hexane diol, 1,6-hexane diol, 1,5-pentane diol, 1, 9-nonane diol, diethyl pentane diol, and 2-butyl ethyl propanediol; alicyclic glycols such as cyclohexane dimethanol and spiro glycol; aromatic glycols such as ethylene oxide or propylene oxide addition product of bisphenol compound; polyether polyol such as polyethylene glycol, polypropylene glycol, polybutylene glycol; polyurethane polyol obtained by the reaction of glycol and polyisocyanate compound. In addition to these dihydric alcohols, trivalent or higher polyhydric alcohol such as glycerin, trimethylolethane, trimethylolpropane, pentaerythritol and the like can be used. These polyalcohols can be used singly or as a mixture of two or more. Further, a glycol including ester functionality in the molecular structure such as bishydroxyethyl terephthalate can be used. Among these polyalcohols, ethylene glycol should comprise at least about 60 mol %, preferably more than about 70 mol %, based on the total polyalcohol components. This high concentration of ethylene glycol provides good film hardness and, in the coating to cans, flavor-retaining ability. Further, it is preferred that butylene glycol be used in combination with ethylene glycol. This combination results in excellent physical properties in the resulting film, such as processability. It is assumed that molar quantity of glycol including ester function in molecular adds with mole ratio of minimum unit monomer. In a bishydroxyethyl terephthalate, for example, it calculates as terephthalic acid 1 mol and ethylene glycol 2 mol.

The carboxyl-functional polyester resin (A) has a number average molecular weight of about from 1,000 to 20,000, preferably about from 2,000 to 10,000, and an acid value of about from 10 to 170 mg KOH / g, preferably about from 25 to 75 mg KOH / g. These characteristics provide good processability, flavor-retaining ability, curability and stability. The ratio of the acid and alcohol components, and the specific kind of each component can be adjusted, according to factors known in the art, so that a polyester resin having above property values is provided. In addition, for good dispersibility and curability, the carboxyl-functional polyester resin (A) should have a hydroxyl value of about 10 mg KOH / g or less, preferably 5 mg KOH / g or less, and have a glass transition temperature of about 0.degree. C. or more, and preferably about from 20 to 80.degree. C.

Problems solved by technology

However, recently questions have been raised about bisphenol A, and effort has been directed to coating compositions in which bisphenol A does not elute.

However, the film forming ability and processability for coating the interior surface of cans is inadequate.

In addition, attempts to improve processability often result in reduction of the stability of an emulsion of this type.

However, the water resistance of the resulting coating was not satisfactory.

However, the hardness and physical properties of the resulting coating were inadequate.

In addition, it has been difficult to provide a good balance between a high hardness in the coating s made using the polyester resin and emulsion stability after dispersion in water.

If the amount is less than 1 part by weight, it results in poor film forming ability and smoothness of coating film, while amounts higher than 200 parts by weight causes instability on emulsifying.